The present invention relates generally to wireless radio telecommunication. More specifically, the invention relates to determining a reference power level for diversity handover base stations in the context of downlink transmit power control.
In a cellular communication system, a mobile radio station communicates over an assigned radio channel with a radio base station. Several geographically-dispersed base stations are connected via digital transmission links to a switching node which is typically connected to a gateway that interfaces the cellular communications system with other communication systems. A call placed from an external network to a mobile station is directed to the gateway, and from the gateway through one or more switching nodes to a base station which serves the called mobile station. The base station pages the called mobile station, and assuming the mobile station responds to the page, establishes a radio communications channel. A call originated by the mobile station follows a similar path in the opposite direction (although there is no need for a page for a mobile-originated call).
In a code division multiple access (CDMA) mobile communications system, spreading codes are used to distinguish information associated with different mobile stations or base stations transmitting over the same radio frequency band. In other words, individual radio xe2x80x9cchannelsxe2x80x9d correspond to and are discriminated on the basis of these spreading codes. Various aspects of CDMA are set forth in one or more textbooks such as Applications of CDMA and Wireless/Personal Communications, Garg, Vijay K. et al., Prentice-Hall 1997.
Spread spectrum communications permit mobile transmissions to be received at two or more (xe2x80x9cdiversexe2x80x9d) base stations and processed simultaneously to generate one received signal. With these combined signal processing capabilities, it is possible to perform a handover from one base station to another, or from one antenna sector to another antenna sector connected to the same base station, without any perceptible disturbance in the voice or data communications. This kind of handover is typically called xe2x80x9cdiversity handover.xe2x80x9d Diversity handover may include xe2x80x9csoftxe2x80x9d and xe2x80x9csofterxe2x80x9d handover. During diversity handover, the signaling and voice information from plural sources is combined in a common point using decisions made on the xe2x80x9cqualityxe2x80x9d of the received data. In soft handover, as a mobile station moves to the edge of a base station""s cell, the adjacent cell""s base station assigns a transceiver to the same call while a transceiver in the current base station continues to handle that call. As a result, the call is handed over on a xe2x80x9cmake-before-break basis.xe2x80x9d Soft diversity handover is therefore a process where two or more base stations handle a call simultaneously. xe2x80x9cSofterxe2x80x9d diversity handover occurs when the mobile station is in handover between two or more antenna sectors connected to the same multi-sector base station using a similar make-before-break methodology. There are several advantages associated with diversity handover such as reduced risk of dropped calls, no interruption in speech upon handover, increased gain in downlink signal-to-noise ratio, and greater protection from log normal and multi-path fading since, on average, the convergence from the effects of fading or multi-paths do not occur at the same time.
Because all users of a CDMA communications system transmit information using the same frequency band at the same time, each user""s communication interferes with the communications of the other users. In addition, signals received by a base station from a mobile station close to that base station are much stronger than signals received from other mobile stations located at the base station""s cell boundary. As a result, distant mobile communications are over-shadowed and dominated by close-in mobile stations which is why this condition is sometimes referred to as the xe2x80x9cnear-far effect.xe2x80x9d
The physical characteristics of a radio channel vary significantly for a number of reasons. For example, the signal propagation loss between a radio transmitter and receiver varies as a function of their respective locations, obstacles, weather, etc. As a result, large differences may arise in the strength of signals received at the base station from different mobiles. If the transmission power of a mobile station signal is too low, the receiving base station may not correctly decode a weak signal, and the signal will have to be corrected (if possible) or retransmitted. Accordingly, erroneous receipt of signals adds to the delay associated with radio access procedures, increases signal processing overhead, and reduces the available radio bandwidth because erroneously received signals must be retransmitted. On the other hand, if the mobile transmission power is too high, the signals transmitted by the mobile station create interference for the other mobile and base stations in the system. Ideally, all mobile-transmitted signals should arrive at the base station with about the same average power irrespective of their distance from the base station.
Interference is a particularly severe problem in CDMA systems because large numbers of radio transmit on the same frequency. If one radio transmits at a power output that is too large, the interference it creates degrades the signal-to-interference ratio (SIR) of other received signals, making it more difficult to correctly demodulate those signals. Accordingly, transmit power control (TPC) is important in a CDMA system. In uplink transmit power control, the mobile station attempts to control its transmit power based on the power control messages sent to the mobile station from the base station with the goal of controlling the power level of the signals received at the base station within a relatively small tolerance, e.g., 1 dB, for all mobile station transmissions received at that base station. In downlink power control, the base station varies the power it is transmitting to a mobile station depending on transmit power control messages or commands sent by the mobile station.
A problem with downlink power control is that the single transmit power control command sent from the mobile station to all of the base stations involved in the diversity handover is not received identically. Because there are different paths between the mobile station and each of the base stations, and because different conditions affect each of those paths, the commands received at different base stations have different bit errors. As a result, the transmit power command may be received correctly in one base station and incorrectly in another base station. The result is that the average transmit powers of the base stations involved in the diversity handover (which should either be the same or have a fixed offset) begin to drift away from the desired value(s). As this base station power drift increases, the full diversity gain is not realized. Diversity gain is ideally realized by receiving two or more radio links of equal power. If one link has a higher power than needed, the extra power is interference which decreases the overall capacity of the communications system. If one link has a lower power than it should, there is a loss of diversity gain.
To combat base station power drift, the power transmission level of each base station in the diversity handover may be compared to a power reference established for all base stations in the diversity handover. The difference between the transmit power of each base station and the reference power threshold may then be used to correct the transmit power level of that base station. Because the power correction depends on the difference between the actual transmit power at the base station and the common power reference, the various transmit powers of the different base stations in the diversity handover converge relatively quickly. Thus, even if the transmit power command from the mobile station is received in error in one or more of the base stations, the power correction based on the comparison to the common power reference compensates for such errors to reduce base station drift, obtain full diversity gain, and reduce unnecessary downlink interference.
The reference power level(s) used in compensating for base station power drift is(are) advantageously determined using one or more parameters relevant to the current condition of the diversity handover communication. Rather than setting an arbitrary and static reference power level, the reference power level is set dynamically so that it is relevant to the current conditions of the diversity handover communication. Dynamic and adaptive reference power level setting results in more effective and more efficient downlink power control. Unnecessary power changes, both in frequency and in size, are avoided because the reference power level is more accurately determined for the current circumstances.
Consequently, the present invention includes a power control method for controlling the transmit power from a first base station and from a second base station to a mobile station in a diversity handover communication. The respective transmit powers of the first and second base stations are first determined. A reference power level is then calculated based on one or more of the determined base station transmit power levels. The reference power level is used to regulate an adjustment or correction made to the respective transmit power levels of the first and second base stations during the diversity handover communication. More specifically, each base station calculates a difference between the reference power level and its current transmit power level. A correction step is calculated using that difference, and each base station adjusts its transmit power level using its corresponding correction step.
In one example, non-limiting embodiment of the present invention, a determination is made regarding which of the first and second base stations is a dominant base station for the diversity handover communication. The dominant base station may be the one whose signal is received by the mobile station with a largest signal-to-interference ratio. An average transmit power of the dominant base station is determined and used as the reference power level for the diversity handover communication. In another example, non-limiting embodiment, and average transmit power is determined for both the first and second base stations, and the reference power level is set to a mean of the average power levels of the first and second base stations. In another example, non-limiting embodiment of the present invention, the average transmit power of the dominant or a favored base station is calculated and used as the reference power level for the dominant or favored base station. It is also used to adjust (rather than set) the reference power level of the non-dominant or non-favored base station.